If we look at the purpose of yoga and understand it in its spiritual context, we could distill it as 'opening the body, at the physical and subjective levels'. In this process of opening what is held within these different levels, the holding and in fact what creates the holding itself, is released. This process of the release through opening is in yogic terms purification. There is no way shape or form by which this process can be equated with stretching.
Stretching does not transform. As the word implies implicitly it is simply a forced lengthening. If we look at asana practice in this way the consequence of stretching will of course result in greater flexibility. This type of flexibility however requires perpetual stretching to be maintained. Because there is a force in stretching the natural response of the body is to retract again once that forces is removed with the same amount of force as was applied. This is a basic law of physics.
It is possible that through stretching, the body can be reshaped, the muscles and tendons lengthened. But if we compare this effect to the purpose of yoga, then the purpose is not achieved. Doing yoga by stretching will also have benefits, but they will be limited and short term. Yoga classes where you are exhorted to pushed to the limit, and then some, may leave you in a state of sweaty exhilaration, but like any high you come down. If this was what yoga was really about, then gymnasts and ballerinas, would be yogis and yoginis.
My yoga teacher in Rishikesh continuously warned a naturally flexible student that her flexibility was a barrier to her practice. He also used to say that there were two ways to we could do asana practice. One way was the way of force, which which I am including stretching. He described the way of force as using a sledgehammer. The second way he described as using a chisel, and a hammer. He said the chisel was awareness the hammer was intelligence. Awareness takes consciousness into the body and intelligence follows it and directions it. As I have written Elsewhere what the body holds is the contents of our subconscious. Obviously this means that we do not have conscious awareness of what is held. Yoga as a process of opening and releasing is then the discovery and making conscious what our subconscious holds in our body.
We live in a fast paced got to have it now culture. Sledgehammer yoga may elicit fast results, but the possibility of injury can be attested to by many. If conscious awareness was there could injury occur? I think not.
What then is opening and how do we do it? Well first of all, as what is held in the body is not conscious, it must become conscious, which essentially means we have to discover it. This discovery requires us paying a lot of attention while doing asana, the first 'union' is between awareness and body, then awareness body and breath.
Holding is by definition a contracting. Opening then is a letting go of the holding / contracting that is unconsciously occurring. It is the creation of spaciousness in the body. On a practical level it looks like this. We must in any asana find and not pass the boundary of our bodies capacity. Then we slightly withdrew from that boundary – just a little. Then we breath into the area where we find our boundary. Inhalation is definition expansion, so we breathe space into the boundary of holding. Then as we exhale we direct awareness into the space we have just breathed open, and we keep doing this over an dover again.
This is asana as a meditation practice. This is yoga as an ongoing relationship of discovery with yourself. To practice this way takes patience, perseverance and courage. Courage because sooner or later you will start meeting the portions of yourself you probably did not even remember or know where there. Parts in pain, in darkness. In practicing this way, it is slower in terms of 'results' from the external view, but the results will last forever and you are fulfilling the age old addition to all spiritual practitioners "know yourself".